I certainly agree with most if not all of what JCW says. At undergraduate level due to structural differences the Cambridge course could be considered better, ultimately you take a larger number of subjects and with fewer proscribed subjects (ie not Jurisprudence and Admin) as at Oxford it is possible to give yourself a broader legal education taking subjects which would only be available at Oxford at a postgraduate level. The fact that at Oxford that apart from Mods most subjects are examined entirely on Finals may be ideal for certain academic disciplines where knowledged/skills/subjects develop in a continuous manner; Law however is not such a subject, each paper being discrete, this combined with the fact that law is a quickly developing subject means that your knowledge can rapidly become outdated- so terminal assessment in the oxford manner is not ideal. That said at the postgraduate level the distinction is not so pronounced.
As for between Downing and Clare, I would choose Downing- traditionally it has what could be described as a "better" reputation for law, it also takes a larger number of lawyers than other colleges. The results for Downing also tend to be higher when compared to Clare. This combined with the fact that Mr Virgo and Mr Hopkins are excellent (I have been taught by both- although I am not a Downing man) would lead me to favour it to Clare- although Mr Hopkins is retiring he should still be supervising. Ultimately you should not choose a College on reputation but instead where you think you will fit in and do well, as the college itself will not really affect job opportunities or careers. A First from New Hall would make you far more attractive to employers than a 2.2 from Trinity. You may wish to consider the Colleges which traditionally take a higher proportion of lawyers than others, they are: Downing, Girton and Trinity Hall; each of which I would strongly recommend to someone considering Cambridge.